Control and reporting for combustible gases such as natural gases or industrial gas mixtures is necessary to assure the quality and usefulness of the gas to a user. While this is sometimes accomplished using composition analysis such as from a gas chromatography and by other methods, it is often sufficient to just determine the relative density of the gas as a basis for control and reporting. Natural gas, primarily methane with dilute mixtures of other gases, is a preferred fuel for energy generation or heat generation because it is clean and efficient.
In many countries, gas for use in households and industries is not available from long distance natural gas pipelines. In these instances, industrially produced gases such as propane and butane are mixed for distribution in local distribution systems. In some cases, natural gas is available, but in limited supply. Mixtures of propane and butane are then used for peak shaving of the natural gas supply. In still other situations, gas which is represented to be propane contains butane in some amount. In order to use such industrially produced gases for residential and industrial fuel, it is necessary that the composition and relative density of the gas be controlled. This is accomplished by measuring the relative density of the mixed gases and controlling the proportion of air in the mixture to adjust relative density of the overall mixture. This prevents the gas from being supplied to customers in a mixture that is too rich or too lean.
In such blending systems, mixing to a consistent Wobbe Index (the ratio of heating value to the square root of relative density) is the operating goal. When the supply gas relative density is known, relative density and heating value are sufficiently related to make it possible to control the blending of the product using only the relative density measurement.
Many local distribution systems for such gases operate at pressures of just 5 to 6 psig. Therefore, whether the fuel gas mixture includes methane or includes propane, butane and air, the relative density measurements should be made at that pressure or a lower pressure to avoid requiring an expensive gas compressor.
The measurement of the relative density of gases has been carried out using several methods and instruments. One instrument has a construction similar to a laboratory balance for measuring a ratio of the weight of a sample gas to the weight of air. The relative density of gases is related to air which is assigned a relative density of 1.0.
Another device for measuring relative density spins a volume of gas and a volume of air in sequential fashion and measures the weight in known volumes. Spinning enhances the sensitivity of the instrument since rotational acceleration increases the forces involved in the weight measurement.
Kennedy, U.S. Pat. No. 4,677,841, discloses a method and apparatus for determining relative density in which a small orifice is formed by a pore of 0.0025 inches in a sapphire jewel. Gas is flowed through the orifice and the rate of change of pressure across the orifice and used to calculate relative density. Kennedy disclosed that at a certain magic pressure, about 11 to 13 psig, the discharge coefficient of the pore can be ignored. While this is an acceptable method for methane and mixture of dilute fractions with methane, which have a relative density less than 1.0 (less than air), it has not proved suitable for use with propane and butane, which have a relative density greater than 1.0 (greater than the density of air). In addition, the method is subject to short term uncertainties because of the use of rate of change for measurement. Blending requires quick response to changes to maintain stability in control.